Hypertrophic cardiomyopathy (HCM), an autosomal dominant disorder characterized by histopathologic findings myocyte disarray and fibrosis, has clinical manifestations of unexplained left ventricular hypertrophy (LVH), diastolic dysfunction, and an increased risk for sudden death. It is a common genetic cardiovascular disorder, affecting approximately 1 in 1000 individuals in the general population. The identification of sarcomere gene mutations as the molecular basis of HCM differentiates this disorder from other types of inherited or secondary causes of LVH. Despite increasingly sophisticated understanding of the causal genetic defects, the precise phenotypic manifestations remain relatively poorly understood. Collaborative basic science and human clinical studies to define the full spectrum of the HCM phenotype are required before molecular discoveries can be effectively translated into the practical management of disease. Genetic-based diagnosis allows for early identification of individuals at risk for developing HCM, prior to the expression of typical clinical manifestations (e.g. LVH). Greater understanding of the preclinical phase of this condition may ultimately inspire rational treatment strategies that will move from contemporary symptom palliation to altering the natural history of disease. Precise identification of early clinical markers of genetic disease will also provide benchmarks for monitoring treatment efficacy. This 2-part longitudinal study proposes to examine and manipulate the early stages of HCM. First, a genotype-positive preclinical population will be examined to detect early alterations in contractile function and myocardial architecture incorporating serial echocardiography with Doppler tissue and strain analysis, gadolinium-enhanced cardiac magnetic resonance imaging, and assessment of serum biomarkers of hemodynamic stress. Second, a pilot interventional trial will be performed to assess the tolerability and efficacy of diltiazem treatment of this preclinical population in improving parameters of diastolic function and to serve as the basis for future larger-scale trials of efficacy.